1. Field of the Invention
The present invention relates to a high-performance, highly-reliable semiconductor device in which an adhesive used to mount (e.g., flip-chip mount) a semiconductor chip on a substrate has less air bubbles therein, and to a low-cost, efficient method for manufacturing the semiconductor device.
2. Description of the Related Art
Flip chip mounting has conventionally been utilized as a method for mounting a semiconductor chip on a substrate, because it can simply the manufacturing process and thus can realize short time, low cost semiconductor mounting. The following, for example, is a known flip chip mounting method: An adhesive is previously supplied to the substrate, convex bumps made of, for example, gold or copper are formed on the electrode pads of the semiconductor chip, the semiconductor chip is mounted face down on the substrate to allow its bumps to face the corresponding electrode terminals of the substrate, a load is applied to the semiconductor chip to allow the bumps to be electrically connected to the electrode terminals, and the adhesive is cured. In this way the semiconductor chip is connected (mounted) on the substrate.
Since bumps can be readily formed with such a mounting method, it is possible to realize low-cost manufacturing of a low-profile semiconductor device in which a semiconductor chip that has a relatively small number of electrode pads—from several tens to several hundreds—is used. For this reason, flip chip mounting is widely applied to digital household electrical appliances such as cellular phones, digital still cameras, and flash memories.
In the flip chip mounting method the convex bumps are referred to as “stud bumps”, each composed of a base part and a protruding part on the base part. With a so-called ball bonding process using a metal wire made of, for example gold or copper, metal balls are secured to the electrode pads of the semiconductor chip by a pressure welding process or pressure welding process using ultrasonic. Note that the protruding part on the base part may be subjected to a planarizing process on an as-needed basis for planarization.
The adhesive is referred to as an underfill material, and is charged in the clearance between the semiconductor chip and the substrate. For the adhesive, for example, insulating adhesives made of epoxy resin, and anisotropic conductive adhesives obtained by adding conductive particles in insulating resins such as epoxy resins are used. The semiconductor chip and the substrate can be bonded together by curing the adhesive, ensuring electrical connection between the bumps of the semiconductor chip and the electrical terminals of the substrate. In addition, sealing the connections between the bumps and electrical terminals as well as circuit elements of the semiconductor chip provides protection to them. The charging of the adhesive may be carried out by previously placing it between the semiconductor chip and the semiconductor substrate upon flip chip mounting as described above, or may be carried out by injecting it in the clearance between the semiconductor chip and the semiconductor substrate after electrically connecting the bumps to the electrode terminals with a flip chip mounting method.
Incidentally, along with the recent demand for downsizing of semiconductor devices and greater packaging density, the size of the electrode pads of the semiconductor chip and electrode pad pitch have been increasingly reduced for the purpose of reducing semiconductor chip size or increasing the number of electrode pads of the semiconductor chip. Accordingly, a semiconductor device manufactured with the flip chip mounting method also has fine bumps on electrode pads when both the size of the electrode pads and electrode pitch are reduced.
Fine bumps, however, cause reduction in the contact area of one electrical connection, and stress concentration on that electrical connection, which is caused due to the difference in thermal coefficient between the semiconductor chip and semiconductor substrate, becomes prominent. For this reason, rupture may take place at the electrical connections when the semiconductor chip is flip-chip mounted on the substrate and, even when rupture has not taken place at this point, residual stress may be concentrated on the electrical connections, resulting in reduction in the reliability of a finished semiconductor device itself.
To avoid this problem, semiconductor devices have been proposed in which a semiconductor chip having a plurality of stud bumps formed on each electrode pad is mounted face down on a substrate to improve reliability of electrical connections (see Japanese Patent Application Laid-Open (JP-A) No. 10-233401, 11-307581, and 2000-286295). Such semiconductor devices offer improved reliability at connections because stress exerted on one connection can be dispersed.
The formation of fine bumps requires use of a metal wire with a smaller diameter. However, as the wire diameter decreases, so too does the diameter of balls that are used in a ball bonding process and the height of bumps to be formed. Thus, when a semiconductor chip is mounted on a substrate, the clearance formed between them becomes small, leading to a reduction in the fluidity of the adhesive injected therein during or after the flip chip mounting. For this reason, air bubbles generated as a result of flow of the adhesive are not fully removed to the outside of the semiconductor chip; they are entrapped in the adhesive. In particular, it is likely that air bubbles are entrapped in the adhesive in the vicinity of bump connections because fine concave and convex shapes are formed there, making it extremely difficult to charge the adhesive in the clearance without inclusion of air bubbles. For example, when a plurality of fine stud bumps are formed on each electrode as in the case of the semiconductor devices disclosed in Japanese Patent Application Laid-Open JP-A) No. 10-233401, 11-307581, and 2000-286295, the space between adjacent stud bumps (in particular the space between adjacent base parts of the stud bumps) becomes very small. Thus, this problem becomes more prominent in such semiconductor devices. When the semiconductor device is mounted on a mother board or the like by reflow soldering, moisture in air bubbles entrapped in the adhesive cause an explosion to cause bulge and/or peeling off of the adhesive, triggering electrical continuity failure at the bump connections in some cases. Moreover, when air bubbles are present in the vicinity of the bump connections, moisture and/or impurities (e.g., ions) in the air bubbles cause current leakage between the adjacent bumps, leading to a reduction in the characteristics of the semiconductor device and to semiconductor device malfunction in some cases. Thus, air bubbles entrapped in an adhesive adversely affects the reliability of the semiconductor device.
A high-performance, highly-reliable semiconductor device in which an adhesive used to mount (e.g., flip-chip mount) a semiconductor chip on a substrate has less air bubbles therein, and a low-cost, efficient method for manufacturing the semiconductor device have yet been provided. In particular, there is a demand for the development of a technology that can reduce air bubbles in the adhesive in a semiconductor chip having minute bumps at a narrow bump pitch.
It is an object of the present invention to solve the foregoing problems and to achieve the object described below. Specifically, it is an object of the present invention to provide a high-performance, highly-reliable semiconductor device in which an adhesive used to mount (e.g., flip-chip mount) a semiconductor chip on a substrate has less air bubbles therein, and a low-cost, efficient method for manufacturing the semiconductor device.